[https://nvbugs/6657468][fix] Set UCX_TLS for GB300 in disagg logprobs test

[moraxu]

Description

disaggregated/test_disaggregated.py::test_disaggregated_logprobs_serving[llama-3.1-8b-instruct] aborts on GB300 during executor init:

[_torch][RANK 0] BindingsNixlTransferAgent init: ... UCX_TLS=tcp UCX_LOG_LEVEL=
W ucx_utils.cpp:631] 4 NVIDIA GPU(s) were detected, but UCX CUDA support was not found!
E ucx_utils.cpp:585] VRAM memory is detected as host by UCX.
E nixl_agent.cpp:545] registerMem: registration failed for the specified or all potential backends
[executor][RANK 1] Failed to initialize executor on rank 1:
  Assertion failed: status == NIXL_SUCCESS (.../nixl_utils/transferAgent.cpp:505)
RuntimeError: Executor worker returned error

Cause. Some Slurm clusters inject UCX_TLS=tcp into the container from the host MPI stack. llm_venv._new_env is an os.environ copy (tests/integration/defs/local_venv.py:42), and setup_disagg_cluster() passes env straight through to the trtllm-serve worker subprocesses, so that value lands in the workers. tcp alone leaves UCX with no CUDA memory domain open, so it classifies VRAM as host memory and NIXL cannot register the KV cache. The UCX_TLS=tcp in the log above is read from os.environ by tensorrt_llm/_torch/disaggregation/nixl/_agent_cpp.py:126 — nothing in TRT-LLM sets it.

Every other setup_disagg_cluster() call site in this file pins the transport list with get_ucx_tls(), which returns cuda_copy,cuda_ipc,sm,self,tcp on SM103 aarch64. This test had that line too, until 285df75 (#16614, "Consolidate dis-agg E2E Tests") removed it — in the same hunk that switched the config from disagg_config_ctxtp2_gentp2_llama31_8b_ucx.yaml to ..._llama31_8b.yaml, i.e. from the UCX backend to backend: DEFAULT → NIXL. The test moved onto the NIXL path in the very commit that dropped the transport pin protecting it.

Fix. Restore the one line. TRTLLM_USE_UCX_KVCACHE=1 is deliberately not restored: the move to NIXL in #16614 was intentional, and re-adding it would flip the test back to UCX and stop exercising the NIXL path.

The repo already documents this failure mode as a known cluster hazard — jenkins/scripts/slurm_env_setup.sh unsets UCX_TLS when it is tcp, with the comment "That disables CUDA transports and breaks NIXL GPU memory registration." That workaround only covers Slurm perf-sanity jobs, not this QA path.

Test Coverage

disaggregated/test_disaggregated.py::test_disaggregated_logprobs_serving[llama-3.1-8b-instruct]

Reproduced and verified on a 4×GB300 node (SM103, aarch64) where the cluster does inject UCX_TLS=tcp, using a locally built SM103 wheel:

	UCX_TLS reaching the NIXL agent	NIXL registration errors	Result
Before fix	tcp	18	FAILED at transferAgent.cpp:505
Control — test_disaggregated_benchmark_gen_only_insufficient_kv, same shell, same NIXL backend, already sets UCX_TLS	cuda_copy,cuda_ipc,sm,self,tcp	0	PASSED
After fix, run 1	cuda_copy,cuda_ipc,sm,self,tcp	0	past executor init
After fix, run 2	cuda_copy,cuda_ipc,sm,self,tcp	0	past executor init

The control run isolates the variable: same node, same shell, same inherited UCX_TLS=tcp, same setup_disagg_cluster() entry point and NIXL backend — it differs only in setting UCX_TLS, and it passes.

The container's UCX is not at fault: it is built --with-cuda=/usr/local/cuda and ucx_info -d reports cuda_cpy (with detect) and cuda_ipc memory domains. UCX_TLS=tcp was filtering them out.

Note on the waive

The GB300 waive is repointed, not removed:

-full:GB300/...::test_disaggregated_logprobs_serving[...] SKIP (https://nvbugs/6657522)
+full:GB300/...::test_disaggregated_logprobs_serving[...] SKIP (https://nvbugs/6275959)

With the NIXL abort cleared, the test now reaches the logprobs comparison and fails there instead:

AssertionError: [completions] logprob mismatch at 1: -0.8889457583427429 vs -0.9349340200424194

That is a streaming vs non-streaming discrepancy, unrelated to KV-cache transport, and it is deterministic — two consecutive runs produced bit-identical values, which rules out flakiness or a too-tight tolerance. It is the pre-existing defect already waived on A100, B200, B300, GB200, H100, H20 and L40S under nvbugs/6275959, first filed 2026-06-08, before this regression. Removing the GB300 waive outright would turn CI red for a bug this PR does not fix, so GB300 is instead brought in line with the other seven platforms.

nvbugs/6275959 therefore remains open and still gates this test everywhere; this PR unblocks the NIXL path underneath it.

PR Checklist

• Please check this after reviewing the above items as appropriate for this PR.

Dev Engineer Review

• The test now sets env["UCX_TLS"] = get_ucx_tls() before starting the disaggregated cluster.
• This restores CUDA-capable UCX transports and prevents NIXL KV-cache registration failures on GB300.
• The waiver now references NVBUG 6275959.
• The changes are limited in scope and use valid existing configuration values.
• No API or public entity changes are present.

QA Engineer Review

• Modified test: test_disaggregated_logprobs_serving[llama-3.1-8b-instruct].
• The test is covered by the updated entry in tests/integration/test_lists/waives.txt.
• Testing on a 4×GB300 node confirmed successful NIXL agent initialization.
• The test reaches an existing deterministic streaming/non-streaming logprobs mismatch.
• Verdict: sufficient.

[coderabbitai]

Walkthrough

The disaggregated logprobs integration test now sets UCX_TLS with get_ucx_tls(). The GB300 waiver entry now references NVBUG 6275959.

Changes

Disaggregated logprobs test

Layer / File(s)	Summary
Test configuration and waiver tests/integration/defs/disaggregated/test_disaggregated.py, tests/integration/test_lists/waives.txt	The test configures the worker environment with the architecture-appropriate UCX_TLS value. The GB300 waiver points to NVBUG 6275959.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 4ba81

The change restores the required UCX transport settings for the GB300 NIXL test and prevents executor initialization failure; no actionable merge-blocking risk remains beyond normal checks.

Suggested reviewers: brnguyen2

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the GB300 failure, its UCX_TLS cause, the fix, the intentional NIXL behavior, test coverage, and the waiver update. It includes all required template sections and a co…
Title check	✅ Passed	The title clearly identifies the NVBugs ticket, fix type, GB300 scope, UCX_TLS change, and affected disaggregated logprobs test.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (1 skipped: 1 …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description clearly explains the GB300 failure, its UCX_TLS cause, the fix, the intentional NIXL behavior, test coverage, and the waiver update. It includes all required template sections and a completed checklist.

Full details: Docstring Coverage

Explanation

Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (1 skipped: 1 unsupported.)

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@tests/integration/defs/disaggregated/test_disaggregated.py`:
- Line 4159: Add test_disaggregated_logprobs_serving[llama-3.1-8b-instruct] to
the appropriate four-GPU test-db CI list, preserving the existing test-list
format and QA entry.

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• tests/integration/defs/disaggregated/test_disaggregated.py
• tests/integration/test_lists/waives.txt

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[moraxu]

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